Adjuvant systemic therapy has been shown to be effective in reducing breast cancer mortality. The additional effect of mammography screening remains uncertain, in particular for women aged 40-49 years. We therefore assessed the effects of screening starting between age 40 and 50, as compared to the effects of adjuvant systemic therapy. The use of adjuvant endocrine therapy, chemotherapy and the combination of endocrine- and chemotherapy, as well as the uptake of mammography screening in the Netherlands was modeled using micro-simulation. The effects of screening and treatment were modeled based on randomized controlled trials. The effects of adjuvant therapy, biennial screening between age 50 and 74 in the presence of adjuvant therapy, and extending the screening programme by starting at age 40 were assessed by comparing breast cancer mortality in women aged 0-100 years in scenarios with and without these interventions. In 2008, adjuvant treatment was estimated to have reduced the breast cancer mortality rate in the simulated population by 13.9%, compared to a situation without treatment. Biennial screening between age 50 and 74 further reduced the mortality rate by 15.7%. Extending screening to age 48 would lower the mortality rate by 1.0% compared to screening from age 50; 10 additional screening rounds between age 40 and 49 would reduce this rate by 5.1%. Adjuvant systemic therapy and screening reduced breast cancer mortality in similar amounts. Expanding the lower age limit of screening would further reduce breast cancer mortality.